Base Structure and Pier Page
The base structure
frame is a 10 sided regular polygon (decagon), 1 meter
The dome will be 3.3 meter in diameter and the base
structure will be slightly less in
diameter, allowing 3-4 cm of dome
overhang for better rain run off. A 10 sided regular polygon with 1
meter sides has a 1.62 meter circumradius (Regular Polygon
The below drawing is a prototype drawing, showing how
the finished observatory will look.
observatory will be located in the back corner of my yard.
fabricated a simple swing arm to simplify staking out the footprint. I
took a 3 meter wood dowel and drilled a hole 1.62 meters from the end.
I nailed this to a wood stake. The wood stake is driven into the ground
in the center of the observatory. Staking out the
footprint then requires only about 10 min. The idea is to place a stake
at the end of the swing arm, then rotate the swing arm and measure out
a 1 meter chord. The intersection of the chord and swing arm gives the
next stake location. The below figure shows the layout procedure.
The stone ring marks an unused concrete lined pond; the
will be integrated into this concrete structure
give greater stability. The base
structure is orientated so that all
the framing contacts the ground outside of the concrete pond,
so vibrations won't be transferred to the telescope pier.
The above photo shows the outline of a concrete lined goldfish pond,
which I had previously filled in with dirt. I decided to remove the
dirt and cement the telescope pier
into and through this concrete
structure. I also filled in the structure with as many large stones as
I could find, to increase its stability. The below photo shows the
concrete pond before pouring the pier. I chiseled a hole through the
concrete. Several 1 meter threaded M12 rods were inserted through the
left hole, and concrete was poured into the large right hole and into
the bottom of the concrete pond. This locked the pier into both sides
of the concrete pond liner, creating a very stabile pier
I built a
simple jig to hold the threaded M12 rods
parallel, inserted them into and through the hole in the
pond, and leveled the jig (below left photo). I poured the
concrete base and then placed a 30 cm x 39 cm form over the rods to
form the pier top.
The form was just a plastic storage container, which
was very smooth plastic that easily slipped free of the dried concrete.
The plastic form was filled with concrete and allowed to dry (below
After sitting for 1 day at 30 c, the plastic form was removed (below
photos). I will cover the pier
with a thin layer of concrete to fill in
The telescope pier foundation was originally built as a low
accommodate my homebuilt 10" GoTo
telescope. I later
purchased a Celestron
C8 and retrofitted a
taller pier; this pier was a gift from
my neighbor and constructed from 6" diameter steel pipe welded to
steel top and base plates (below photo)
pier foundation was not designed to accommodate a tall
metal pier, there were stability problems. The pier acted like a long
pendulum and vibrated too much. I filled the pier with sand and this
partially dampened the vibrations,
but not to an acceptable level. I was planning to weld triangular
struts between the base plate and tube, but
realized there was another problem: the
base plate had bent.
I encased the pier bottom in a large concrete block
molded holes to accept the pier foundation bolts. This firmly locked
the metal pier to the concrete pier foundation and strengthened the
pier bottom against bending. The concrete mold was a Styrofoam cooler
(below photo). I used wood dowels to form the channels for the pier
foundation screws and coated the Styrofoam and wood dowels with cooking
oil (raps oil) as a mold release agent.
The mold and
metal pier were leveled, the Styrofoam mold was
filled with concrete, and a few sections of rebar
placed around the metal pier. I used an old paint bucket to
the top cylindrical concrete section. The paint bucket was also coated
with raps oil and the rebar extended up into the cylindrical concrete
section (below photo). I used a slightly dryer concrete mix for
the cylindrical section to reduce leaking between the paint
and rectangular concrete sections.
was left to set overnight, removed from the mold, and the
oil coated wood dowels were easily hammered out of the concrete.
Below are two photos of the finished pier in my observatory.
concrete pier base has significantly strengthened the pier and greatly
The base structure
is a 10 sided regular polygon with 1 meter sides;
this will give a pretty good approximation of a circle when it's clad
with exterior siding. The base
structure is built around the telescope
direct contact between the pier
and observatory. The
basic design was modeled after a photo on Clement's
I filled in the space around the concrete pier with sand and
the pier with a
ring of masonry blocks (below photo). The floor joists
will rest on the masonry blocks and the sand will hopefully help to
isolate the pier
from any vibrations.
leveling the masonary ring, I dug holes where the 10 polygon
sides intersect (previously staked out). These holes were filled with
sand and a masonry block was added to support the framing. I dry fit
the floor joists and adjusted all masonry blocks to level the joists
(below photo). Additional framing photos can be found on the Observatory
Photos web page.
progress (below photo). The top horizontal cross supports
(3.8 cm x 5.7 cm x 1 m) form a 72 degree angle with the radius
I set my miter saw at 18 degrees and cut all cross braces and
exterior floor joists.
The 10 upright posts (7.2 cm x 3.6 cm x 1 m) and all
floor joists (4.5 cm x 9.5 cm) rest on the masonry blocks. The
bracing between the 10 upright posts and the perimeter floor
are 3.8 cm x 5.7 cm x 55 cm, with a 45 degree bottom miter and
45 degree x 18 degree compound top miter.
The floor will
sit on the 10 radial floor joists and the 10
perimeter floor joists (below left). I installed two
additional concentric rings of floor joists, each set 40 cm on
center. The below left and right diagrams show the base structure
floor joist plan and side plan, respectively.
structure was connected to 4 pressure treated posts,
anchored in concrete and located around the outer perimeter (below
photo). If I ever want to move the structure, it can then be easily
disconnected from the posts without removing the
flooring. I replaced the temporary screws with M8
exterior bolts when I sided the base structure.
have an additional plywood base
top braces to increase stability. I will fabricate a
plywood ring to support the dome (dome
ring), but I wanted to
using an additional plywood base
ring on top of the top braces. My solution
was to strengthen the top braces by adding corner supports at all 10
uprights. The corner supports are just scraps of 3.8 cm x 5.7 cm wood,
cut to a 144 deg. angle, that fit into the internal angles where the
top braces meet (below photo). This greatly strengthened
the top braces and
eliminated the need for an additional plywood base ring.
The original plan was to use heavy plywood for the flooring, but this
would cost too much; a quick calculation showed that 8 m2
of 20 mm thick plywood would cost in excess of $200. This clashed with
my design theme of keeping everything simple and inexpensive. I
determined that 80 linear meters of 25 mm x 12 cm pine boards would
cost about 50% as much as 8 m2 of
plywood. I purchased
9 linear meters of 25 mm x 10 cm pine boards for a test fabrication, as
shown in the below photo.
25 mm pine
was thick enough for most of the flooring, with the
exception of the three longest boards at the outer perimeter. I
installed 10 additional cross joists perpendicular to the outer
perimeter (marked with arrows in the above photo), which gave
satisfactory support to the 25 mm pine. There is 3-4 mm
between boards, to allow for expansion. The below photo shows
finished observatory floor. The floor boards were treated with an
exterior oil based water sealant and secured with 55 mm
The exterior siding is 6 mm thick plywood (122 cm x 244 cm sheeting).
I treated the plywood with an oil based water sealant and let
dry overnight. The 6 mm plywood easily bends around the 10 sided frame,
giving a pretty good approximation to a circular base structure.
plywood is fastened with galvanized screws .
I added a rain skirt above the door and around the door lock
to keep the door weathertight (below photo).
The above photo shows that there is a small gap between the observatory
bottom and the ground, allowing wind to blow under the observatory and
up around the telescope pier. I added a 15 cm wide polyethylene wind
barrier to the base structure (below photo). The polyethylene film is
tacked to the exterior siding and secured at the top with a strip of
plastic floor molding and silicone caulking.
Stone Rain Ring
After installation of the base structure wind skirt, I added
ring of masonry block (below photo). The masonry block serves
several functions: securing the wind barrier, making it more difficult
for rodents to tear through the polyethylene wind barrier, and
absorb the impact from rain running off the dome overhang. All masonry
blocks were angled to channel rain run off away from the observatory.
The dome ring
will sit on 10 dome
bearings that can be
adjusted (raised or lowered).
I purchased a single roller and made a
prototype (below photo). I drilled out the mounting bracket holes to 6
mm and connected two M6 threaded rods. The M6 bolts and
allow the roller to be raised or lowered relative to the base
This roller is rated for a 20 kg load, giving a maximum
limit of 200 kg for the dome.
I eventually found a larger wheel that had a M6 hole and was rated
for 25 kg. It is important that the dome bearings
are placed tangent
the base structure
and dome ring.
Because I had installed corner supports, this
simplified the layout procedure. Since each corner support is fitted to
be tangent to the base
structure, I could use this to aline the dome bearings. I
positioned the dome
bearings where I wanted them and used a carpenter's square
to aline the wheel bracket tangent to the base structure and parallel
to the corner support (below left photo). The below right photo shows
an installed dome
bearing. There are M6 washers and bolts below and
the wheel bracket. This allows each wheel to be independently raised or
lowered to level the dome
ring. An additional photo of the dome ring sitting on a dome
bearing can be found on the Observatory
Photos web page
bearings serve two functions: they keep the dome centered
on its axis of rotation and prevent the dome from lifting off of the
The prototype centering
bearings were constructed from
steel plates and brackets (below photo). The idea was
steel plates to flex as the dome rotated, however they were too weak
and bent inwards; otherwise the basic bearing
design (below photo) just replaced the steel plate with left
over floor joist scraps (4.5 cm x 9.5 cm).
photo shows the final centering
bearing; note the hole that
was cut in the arch
support to create a lip and remove obstructions to
the top bracket with the small roller.